The discovery of the signal transduction pathways that activate cell proliferation in response to interactions between growth factors and corresponding cellular receptors, triggered an extensive search for inhibitors that can interfere with this cascade in malignancies where malignant cells undergo uncontrolled proliferation. The chemical signals in this cascade have been identified as phosphorylation of proteins either on tyrosine residues, catalyzed by a group of enzymes collectively termed protein tyrosine kinases (PTK), or on serine/threonine residues by protein kinases A, B, and C. Protein kinase C (PKC) is also an important cellular signal transducer that contains a catalytic domain which phosphorylates substrates and a regulatory domain which controls its activity. Polyhydroxylated flavones such as genistein and quercetin were identified as inhibitors of the phosphorylation kinases (Losiewicz et al, 1994).
Perylene quinones are a unique group of kinase inhibitors (Diwu et al, 1994). The first of these compounds to be thoroughly evaluated was hypericin, a potent photodynamic agent initially discovered by the present inventors to be virucidal to retroviruses (Lavie et al, 1989; Meruelo et al, 1988), and subsequently to all lipid-enveloped viruses (Tang et al, 1990). Additional studies identified hypericin as a potent and irreversible light-dependent inhibitor of protein kinase C (PKC), particularly when PKC is translocated to the cell membrane following cell activation, this PKC inhibitory activity of hypericin being possibly related to its antiretroviral activity (Takahashi et al, 1989).
Hypericin is able to act within biological systems in the dark, possibly because of a low red/ox potential, and this appears to enable electron scavenging from physiological transfer reactions (Lavie et al, 1994). The unique combination of properties of hypericin prompted its current clinical evaluation in phase II clinical trials as an anti-tumor agent in the treatment of malignant glioma (Couldwell et al, 1994). This neoplasia relies on PKC signaling for cell proliferation. Hypericin is also a potent photosensitizer capable of generating singlet oxygen and free radicals (Hadjur et al, 1994). These properties also render it useful in photodynamic therapy (PDT) of superficial tumors accessible to light irradiation.
Unfortunately, hypericin is active in only half of the cases and, in addition, may cause severe side effects, such as prolonged post-treatment sensitivity to light, a condition medically known as hypericism. It would be desirable to provide additional photosensitizing agents and cell proliferation signal transduction inhibitors which can elicit their cytotoxic effect with greater efficiency as compared with existing agents and, potentially, with lower and less severe side effects.